Exhaust gas emissions from traffic, particularly road traffic, constitute a significant part of harmful emissions inflicted by human activity on the environment. In addition to the detrimental gaseous emissions, such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx), the fine particles contained in exhaust gases from vehicles have been found to be a significant health hazard, and therefore, certain upper limits for fine particle emissions have also been set by legislation. The trend is to continuously and gradually reduce said limit values and simultaneously to bring them to a uniform international level. Authorities setting the limit values for exhaust gas emissions include, for example, the European Commission and the United States Environmental Protection Agency (US-EPA).
Simultaneously with the development of combustion engine technology with the aim of always reducing the exhaust gas emissions, the control and diagnostic systems of the engines become more and more significant. A common concept in the field is so-called OBD (On-Board Diagnostics) sensors, which refer to sensors controlling the control systems of a vehicle and/or monitoring the function of certain components in the vehicle.
OBD sensors will also be increasingly used to control that particle emissions from the exhaust gases of vehicles remain within allowed limit values. A particular application in view of this invention is the checking, indication and/or control of the function of so-called particulate traps used in the exhaust gas systems of vehicles. These particulate traps are used especially in the exhaust gas systems of Diesel vehicles. Having collected particles for a given time, such a particulate trap requires so-called regeneration, in which regeneration the particles contained in the particulate trap are typically burnt by methods known as such, for example by increasing the temperature of the particulate trap and by simultaneously introducing into the particulate trap a sufficient quantity of air required for burning the soot particles.
U.S. Pat. No. 4,939,466 presents a measuring method and a device for indicating the operation of renegenerating the particulate trap of a Diesel vehicle. The arrangement is based on the use of an electrical charge sensor placed after the particulate trap in the exhaust duct. The particles developed during the regeneration have a natural electrical, typically positive charge which can be detected by a sensitive charge sensor placed in the exhaust duct. According to the teachings of said reference, the charge sensor can be an inductive annular electrode, or it may also have a net-like structure. The sensor is capable of detecting when the regeneration of the particulate trap occurs.
U.S. Pat. No. 4,456,883 presents a method and a device for determining the operation of a combustion engine by measuring particle emissions from the exhaust gases. The measuring arrangement comprises an electrostatic sensor placed in the exhaust duct to detect the periodic particle emissions from the different cylinders of the engine on the basis of the natural charge obtained by the particles in connection with the combustion. The cyclic measurement result can be averaged to determine the average particle emission, or the measurement result can be analyzed with respect to time to detect problems occurring in the operation of a single cylinder.
In the above-mentioned techniques for detecting particles under exhaust duct conditions by utilizing the natural charging of the particles, problems are caused by the fact that the natural charging of the particles is dependent on a large number of different factors, such as the quality of the fuel as well as, in a very complex way, the operating conditions and the loading of the engine. Furthermore, when a particulate trap or filter is used before the measuring point, the measurement result is also affected by the fact that the particulate trap or filter, as such, affects the electric charge of the particles passed through in a very complex way.
Solutions are also known from prior art which do not rely solely on the natural charging of particles contained in the exhaust gas but which use a separate charger for charging the particles before the measuring point.
JP 63255651 discloses a particle measuring sensor in which particles contained in exhaust gas are charged by a cathode electrode, and when they further impinge on an anode electrode, their electrical charge is detected on the basis of a sensitive current measurement.
JP 60100046 discloses a particle measuring sensor placed in an exhaust duct and comprising charging electrodes placed before measuring electrodes in the flow direction of the exhaust gas. By the high-voltage charging electrodes, particles contained in the exhaust gas are negatively charged before they are detected by the measuring electrodes.
However, when a separate charger is used according to the prior art for charging the particles, the following problems will be caused in practice.
In principle, the exact measurement of the number of the particles will require that also the charge of the particles must be known exactly. In the solutions of prior art, the measurement result will be affected by the natural charge obtained by the particles in connection with the combustion, because said charge is added to the charge obtained by the particles in the charger. This will cause a significant inaccuracy in the measurement result. The effect of the original, natural charge of the particles can be reduced by using sufficiently long retention times as well as a sufficiently high charge density for the particles in the charger, wherein all the particles are charged substantially in the same way, achieving a balanced charge. In practice, however, the result is that impractical large-size and powerful chargers must be used in vehicles. Furthermore, such powerful chargers act, as such, partly in the manner of an electrical filter and tend to remove some of the particles to be measured from the range of the charger. Thus the particles accumulated in the charger induce both a measuring error and the soiling of the charger.
In all cases, for measuring particle emissions on the basis of the electrical charge of the particles after a particulate trap or filter placed in the exhaust duct, one must measure relatively small currents, typically in the order of picoamperes, due to the relatively small number of the particles. This is difficult to implement under the demanding conditions of the exhaust duct, particularly for the following reasons. First of all, the temperature of the exhaust gases is high, typically several hundreds of degrees centrigrade at the measuring point, which causes significant leakage currents in the insulators of the electrodes placed in the exhaust duct and, further, significant current noise and drift of bias currents due to the elevated temperature of the components at the measuring amplifier placed in the vicinity of the electrode. Secondly, the exhaust gases contain impurities which soil the insulators. In the long run, the soiling of the insulators will increase leakage currents and noise in the current signal to be measured, thereby causing an increasing error in the measurement results. A quite significant problem in the measuring devices placed in the exhaust duct is the soiling induced by soot particles which may involve both actual clogging of the sensor structure and impairing of the insulation capacity of all the electrical insulators connected to the sensor. Moreover, the soot deposits interfere with the operation of the ionization or charger electrodes.